1. Field of the invention
The present invention relates to a signal power control technology using a variable gain amplifier.
2. Description of the related art
The variable gain amplifier (VGA) is a circuit that variably controls a gain based on a control signal. The variable gain amplifier executes gain control to obtain a gain corresponding to a gain set value called a gain code contained in the control signal. There is a technique of linearly controlling power of an input signal by a power control circuit constructed of a combination of an amplifier, corresponding to this type of variable gain amplifier, capable of finely adjusting a gain adjustment range (which will hereinafter be referred to as a fine amplifier) and an amplifier, corresponding to the variable gain amplifier, having a coarser gain adjustment range than that of the fine amplifier (which will hereinafter be termed a coarse amplifier).
FIG. 16 is a diagram showing a conventional power control circuit. A conventional power control circuit 50 illustrated in FIG. 16 is constructed of a fine amplifier 51, a coarse amplifier 52 and a decoder 53.
FIG. 17 is a graphic chart showing gain characteristics of the fine amplifier 51 and the coarse amplifier 52. The fine amplifier 51 and the coarse amplifier 52 individually amplify the signal to be inputted with the gain characteristics illustrated in FIG. 17. A corresponding relationship between the gain codes inputted to the fine amplifier 51 and the coarse amplifier 52 and the gains, is referred to as a gain characteristic. In FIG. 17, a proportional straight line indicated by AMP. (FINE) on the left side of the graph represents the gain characteristic of the fine amplifier 51, while a black dot on the right side of the graph represents the gain characteristic of the coarse amplifier 52.
The fine amplifier 51 and the coarse amplifier 52 receive control signal (the gain codes in FIG. 17) sent from the decoder 53, and control the gains so as to obtain the gains corresponding to the control signal. According to the example in FIG. 17, the fine amplifier 51, which is supplied with gain codes CF0 through CFm, performs the gain control of the gains from 0 (dB(decibel)) through G1(dB). The coarse amplifier 52, which is supplied with gain codes CC0, CC1, CC2, executes the gain control stepwise such as 0(dB), G1(dB), G2(dB).
The decoder 53 receives the control signal inputted from an external circuit, decodes the control signal, thereby extracting the gain code from this control signal. The gain code sent from the external circuit is a set value representing a request gain, and the power value of the signal output from the conventional power control circuit 50 becomes the power value corresponding to an in-design gain value specified by the gain code. The decoder 53 determines each gain code (which is the gain code shown in FIG. 11) that should be sent to the fine amplifier 51 and the coarse amplifier 52, corresponding to a gain code showing the request gain (which will hereinafter be termed a request gain code). The decoder 53 transmits the thus-determined gain code to the fine amplifier 51 and the coarse amplifier 52, respectively.
FIG. 18 is a graphic chart showing an example of the gain control by the conventional power control circuit 50. According to the example in FIG. 18, if a gain G1x(dB) of the power control circuit 50 is requested, the control signal showing a request gain code C1x is inputted to the power control circuit 50. In the power control circuit 50, the decoder 53 extracts the request gain code C1x from the control signal, and determines the respective gain codes of the fine amplifier 51 and the coarse amplifier 52, which are necessary for acquiring the gain requested with this request gain code C1x. According to the example in FIG. 18, the gain code of the coarse amplifier 52 is determined to be CC1, and the gain code of the fine amplifier 51 is determined to be a code (e.g., CF3) for obtaining the gain (G1x−G1)(dB).
The decoder 53 sends the thus-determined gain code CC1 to the coarse amplifier 52, and also the gain code CF3 to the fine amplifier 51. As a result, as shown in FIG. 18, the output signal controlled with the gain G1x corresponding to the request gain code C1x is output from the power control circuit 50.
According to this type of conventional power control circuit 50, the combination of the two amplifier enables the gain to be finely adjusted with the larger gain range (ranging from 0 to G3 in the example in FIG. 18).
Note that the following document is disclosed as the document of the conventional art related to the invention of the present application. The document is a “Japanese Patent Laid-Open Publication No. H11-177371”.
By the way, the conventional power control circuit 50 might, as illustrated in FIG. 19, in some cases, have occurrence of a difference between the gain characteristic that can be taken in design (in specification) of each of the fine amplifier 51 and the coarse amplifier 52 and the gain characteristic that is taken in an actual control operation. The difference between the gain characteristic that can be taken in design and the gain characteristic that is taken in the actual control operation, will hereinafter be referred to as a misalignment in the gain characteristic.
FIG. 19 is a graphic chart showing an example of the misalignments in the gain characteristics of the fine amplifier 51 and the coarse amplifier 52. The example in FIG. 19 shows that in the fine amplifier 51, when a maximum code CFm is inputted, the gain characteristic depicted by a bold solid line in the actual control operation gets smaller by D1(dB) than the in-design gain characteristic depicted by a long chain line. In the coarse amplifier 52, with respect to each gain code, the gain characteristic depicted by the black dot in the actual control operation is larger by D2(dB) than the in-design gain characteristic depicted by double circles. Thinkable causes of the occurrence of the misalignment in the gain characteristic are a manufacturing scatter of each circuit, a fluctuation in characteristic of each circuit that accompanies a change in temperature, etc.
FIG. 20 is a graphic chart showing an example of the misalignment in the gain control in the conventional power control circuit 50. As in the example in FIG. 19, if the misalignment occurs in the gain characteristic of each amplifier, the misalignment as shown in FIG. 20 occurs in the gain control of the conventional power control circuit 50. At this time, if G1(dB) or G2(dB) is requested as the gain, the request gain code C1m or C2m is inputted.
If such a misalignment in the gain characteristic occurs, however, the circuit is bound to enable the linear control to be done, and nevertheless the conventional power control circuit 50 can not actualize a gain range indicated by a dot pattern area in FIG. 20. This gain-uncontrollable gain range will hereinafter be referred to a gain step.
Accordingly, in the conventional power control circuit 50, a scheme for preventing the gains step from occurring involves combining the gain characteristics of the fine amplifier 51 and the coarse amplifier 52 so as to include a sufficient allowance by taking account of the misalignments described above. Namely, a design aims at increasing an overlapped range of the output gains of the fine amplifier 51 and the coarse amplifier 52.
In the conventional power control circuit 50 taking this design technique, however, duplication appears in the gain ranges that can be taken for the fine amplifier 51 and the coarse amplifier 52. Hence, if the misalignment in the gain characteristic does not occur, such a duplicated gain range is not utilized, and the conventional power control circuit 50 can not be therefore said to be a high-efficiency control circuit. As a matter of course, it follows that the conventional power control circuit 50 gets enlarged in circuit scale with futility.